Static transfer switch with turn off circuit

ABSTRACT

A static transfer switch is provided for supplying power to a load alternately from two different power sources. Switching between the two power sources may occur within a fraction of one electrical cycle. In response to sensing degraded performance in the power source supplying the load, a gate signal is turned off to a first switch coupled between the power source and the load. A third switch coupled between an energy storage and the first switch is also closed to release a current to the input or output of the first switch The current forces a drop in current conducted through the first switch and causes the first switch to open and stop conducting current. A second switch coupled between the alternate power source and the load is then closed to supply power to the load from the alternate power source.

BACKGROUND

The present inventions relate generally to a static transfer switch fortransferring power from one power source to another power source tosupply an electrical load.

Static transfer switches are used in the industry to control theelectrical power supply to critical electrical components. Inparticular, static transfer switches are used for electrical loads likedata centers where a constant, high-quality electrical supply isrequired.

An example of a static transfer switch 10 is shown in FIG. 1. As shown,two different electrical power sources 12A, 12B are coupled to thestatic transfer switch 10. The output of the static transfer switch 10is coupled to an electrical load 14. Typically, the output is directlyconnected to a Power Distribution Unit (PDU) 14, which includes atransformer 16. The final electrical load may be racks of computerservers 30 (FIG. 2) in a data center. However, it is understood thatstatic transfer switches 10 may also be used to supply power to othertypes of electrical loads.

The static transfer switch 10 may include a variety of sensors 18A, 18B,20A, 20B to monitor electrical properties of the first and second powersources 12A, 12B and the power output. For example, it may be desirableto monitor the voltage 18A, 18B of each of the power sources 12A, 12Band to monitor current 20A and voltage 20B of the output. The statictransfer switch 10 also includes one or more switches 22A, 22Bassociated with each of the power sources 12A, 12B. This allows thestatic transfer switch 10 to supply power to the output from either ofthe power sources 12A, 12B. For example, the first power source 12A maybe the preferred power source 12A (e.g., the grid), and the second powersource 12B may be a backup power source 12B (e.g., a generator). Innormal use, power can be supplied from the first power source 12A to theload 14 by closing the first switch 22A and opening the second switch22B (to disconnect the second power source 12B). In the event that thefirst power source 12A suffers from degraded performance (e.g., drop involtage) as determined from one or more of the sensors 18A, 18B, 20A,20B, the power supply can be transferred to the second power source 12Bby opening the first switch 22A and closing the second switch 22B. Thus,the electrical load 14 is provided with a constant source of powerdespite the possibility of degraded performance events in one of thepower sources 12A, 12B.

SUMMARY

A static transfer switch is described for increasing the speed ofswitching from one power source to another power source. The systemsenses degraded performance of the power source supplying power to theload. In response to sensing degraded performance, the system turns offa gate signal to a first switch coupled between the power source and theload. The system also closes a third switch coupled between an energystorage and the first switch to release a current to the input or outputof the first switch The current forces a drop in current conductedthrough the first switch and causes the first switch to open and stopconducting current. As a result, the first switch may be openedsubstantially faster than in conventional static transfer switches.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

The invention may be more fully understood by reading the followingdescription in conjunction with the drawings, in which:

FIG. 1 is a schematic of a static transfer switch;

FIG. 2 is a schematic of the static transfer switch showing three-phasepower sources and a three-phase load;

FIG. 3 is a schematic of a switch circuit coupled between one phase ofthe power source and the load;

FIG. 4 is a series of graphs showing electrical properties of the switchcircuit during opening of the switch between the power source and theload;

FIG. 5 is a schematic of a charge circuit for the energy storages;

FIG. 6 is another schematic of the static transfer switch;

FIG. 7 is a flow chart showing a transfer of power from one power sourceto another power source to supply a load;

FIG. 8 is a schematic of a digital signal processor used for controllingpower through the static transfer switch; and

FIG. 9 is another schematic of a digital signal processor used forcontrolling power through the static transfer switch.

DETAILED DESCRIPTION

An example of a three-phase static transfer switch 10 is shown in FIG.2. Typically, static transfer switches 10 are designed to complete aswitching event between the two power sources 12A, 12B within oneelectrical cycle of the power sources 12A, 12B. This is desirable sothat a high-quality, constant power supply is provided with minimaleffect on the electrical load 14, 30. In order to achieve switchingevents this quickly, it is typically necessary to use solid-stateswitches 22A, 22B (first switches 22A and second switches 22B) toperform the switching event since solid-state switches 22A, 22B can beswitched on and off in less than one electrical cycle. Preferably, theswitches 22A, 22B are silicon controlled rectifiers (SCR). Various typesof thyristors may be used for the solid-state switches 22A, 22B, such asintegrated gate-commutated thyristors (IGCT), reverse blockingintegrated gate-commutated thyristors (RB-IGCT), or gate turn-offthyristors (GTO). In the case of a multiphase static transfer switch 10,each of the main switches 22A, 22B will be made up of multipleindividual switches 26, with at least one switch 26 for each phase A, B,C. A pair of anti-parallel thyristors 26 is particularly well-suited foreach switch 26 associated with a phase A, B, C. Because the powersources 12A, 12B are AC power sources, each switch 26 typically includestwo switches 26A, 26B in an anti-parallel arrangement. However, eachpair of anti-parallel thyristors 26A, 26B are often treated as a singleswitch 26 because they typically turn on and off together. The statictransfer switch 10 may also include a series of manual switches 28 thatare primarily used during maintenance to isolate sections of thecircuit.

It is often preferred for the first and/or second power sources 12A, 12Bto include an uninterruptible power supply (UPS) 32 to provide controlover the electrical properties of the original source 12A, 12B andmanage power drops or losses in the original source 12A, 12B. As noted,the output is typically coupled to the transformer 16 of a PDU 14, andthe final electrical load 30 is often racks of computer servers 30 in adata center.

An improvement of the invention herein is that it uses a resonant turnoff topology adjusted to force commutate a three-phase power system andis operated by an embedded digital processor controlling the signalswith software intelligence and algorithms in order to achieve autonomyand make possible a sub-millisecond transfer switch.

The turn off circuit 40 with RTO topology is shown in FIG. 3. As shown,the circuit 40 includes a main circuit 42 and a resonant circuit 44. Themain thyristors 26A, 26B (first switches) are S_(m1) and S_(m2). Theresonant circuit includes four auxiliary thyristor switches 34 (thirdswitches) S_(r1), S_(r2), S_(r3), S_(r4), resonant capacitor C (energystorage) 36, and resonant inductor L 38. The capacitor C 36 ispre-charged to provide resonant current to create a zero-currentcrossing for the main thyristors 26. The inductor L 38 limits di/dt formain thyristors 26 during turn-off. During normal conduction, only themain thyristors S_(m1) (or S_(m2)) 26A, 26B are conducting and all theauxiliary switches 34 are off. Thus, the pre-charged resonant capacitor36 is isolated from the main thyristor switches 26. In resonant turn-offoperation, the auxiliary switches 34 S_(r1,2) (or S^(r3,4) depending oncurrent direction) are triggered to open by sending a gate signalthereto. As a result, the energy stored in the resonant capacitor 36 isdischarged. When the resonant current through the inductor 38 L exceedsthe load current, the current through the main thyristor 26 is commutedto the resonant circuit. In the meantime, the capacitor 36 voltageprovides a negative bias voltage to help the main thyristor 26 turn off(i.e., open and stop conducting). When the main thyristor 26 currentreaches zero, it starts turning off with reverse bias voltage from theresonant capacitor 36. In the RTO topology, there are three possibledesign choices to control performance: resonant capacitance value C,pre-charged initial capacitor voltage V_(c0), and resonant inductancevalue L. These parameters can be used to determine how much and how fastthe main thyristor current can be turned off, as well as the size andcost of the auxiliary resonant circuit.

FIG. 4 shows the voltage and current waveforms of resonant turn-offoperation for the main thyristors 26 and auxiliary switches 34. As shownin the bottom graph, at time 0.1 seconds a gate signal is sent to turnoff the main thyristor 26, and a gate signal is sent to turn on theauxiliary thyristors 34. This occurs as a result of degraded performancebeing identified by a sensor 18, 20. Because the second switches 22Bcannot be closed to switch power to the second power source 12B untilthe first switches 22A have been opened, it would be beneficial to beable to stop current conduction through the first switches 22A fasterthan in a conventional static transfer switch. That is, once a turn ongate signal has been applied to an SCR, current will continue to beconducted through the SCR even after a turn off gate signal is applied.However, after a turn off gate signal is applied to the SCR, the SCRwill stop conducting current after the current drops below a threshold.That normally occurs with an AC current when the current waveformcrosses zero. However, that may require up to half of an AC cycle forthe power source waveform to cross zero (i.e., 8 ms). It would bebeneficial to be able to stop the first switches 26A quicker so that thesecond switches can be closed more quickly to transfer power as quick aspossible from the first power source 12A to the second power source 12B.As shown in the top graph in FIG. 4, current to the load 14, 16 throughthe main and auxiliary thyristors 26, 34 stops in less than 0.18 msafter applying the gate signals to the main and auxiliary thyristors 26,34, and preferably within 0.5 ms. Thus, the turn off speed is greatlyincreased.

Referring to FIG. 4, after the auxiliary resonant thyristors 34 aretriggered, the resonance starts and the resonant current through L 38increases fast. Once the resonant current is larger than the load 14, 16current, the main thyristor 26 current decreases to zero and startsturning off. In the meanwhile, the load 14, 16 current is commutated orbypassed to the auxiliary circuit 44 (34, 36, 38). The resonantcapacitor 36 is re-charged by the load 14, 16 current but in reversedvoltage polarity. After the resonant capacitor 36 voltage is high enoughand the load 14, 16 current is interrupted, the auxiliary thyristors 34are turned off. As a result, the RTO has disconnected the first powersource 12A and the load 14, 16 is ready to be transferred to thealternative power source 12B.

The resonant circuits 44 (one for each phase) are shown in FIG. 5. Asshown, a pre-charge circuit 46 is coupled to the resonant circuits 44with a DC bus 48. Preferably, the DC bus 48 is pre-charged slowlythrough the single bridge rectifier diodes 50 with resistors 52 to makesure capacitive inrush is prevented. Also, in case the DC bus 48 runsabove the set limit, a bleeding resistor 54 can be used to adjust thatDC level to the desired value. A pre-charge relay 56 and bleed relay 58are used to turn on and off the pre-charging and bleeding. Preferably, afourth switch 60 is provided between the pre-charge circuit 46 and thecapacitors 36 to disconnect the pre-charge circuit 46 from thecapacitors. The fourth switch 60 is preferably opened to stop chargingof the capacitors in response to the switching event. If desired, fourthswitches 60 may be provided for each resonant circuit 44 to disconnectthe respective circuit 44 from the DC bus 48.

A phase locked loop (PLL) 62 (see FIG. 9) is used to determine thefrequency and phase angle of the AC waveform of the power source 12A.The PLL 62 can be used in determining the firing sequence for theauxiliary switches 34 because the switches 34 need to be fired at thecorrect polarity. Since this is a three-phase system, the polarity ofphase A, phase B and phase C will not be the same at the same time. Thefiring sequence is determined by the following algorithm:

-   -   IF 0<=PLL_RADIAN [PHASE_A]<=π        -   Turn ON OUT_RTO_PLUS_A        -   Turn OFF OUT_RTO _MINUS_A    -   ELSE        -   Turn ON OUT_RTO_MINUS_A        -   Turn OFF OUT_RTO_PLUS_A

The same algorithm is applied for the other remaining phases to completethe firing sequence in case a transfer is needed. In other words, onlytwo of the auxiliary switches 34 are turned on to release current fromthe capacitor 36, with one switch 34 being coupled to the input of themain thyristors 26 and the other switch 34 being coupled to the outputof the main thyristors 26. The two auxiliary switches 34 that are turnedon depends on current flow of the AC current through the main switches26. Thus, when the current is positive, one pair of switches 34 will beturned on (while the other pair remains off), and when the current isnegative, the other pair of switches 34 will be turned on (again, withthe other pair remaining off). Preferably, all of the main switches 22A,26 are turned off (i.e., opened) and the auxiliary switches 34 areturned on (i.e., closed) at the same time in the static transfer switch10. Because some phases A, B, C will have a positive current and somephases A, B, C will have a negative current at the switching instant,the pair of auxiliary switches 34 that is closed in each phase resonantcircuit 44 will vary depending on the current of the particular phase A,B, C.

The main circuits 42 are shown in FIG. 6. The table below lists thenames of the signals used in the main circuits 42 and the resonantcircuits 44 of FIGS. 5-6. As explained below, a DSP 64 (FIGS. 2 and 8-9)may be used to generate the control signals used by the main circuits 42and resonant circuits 44.

Name Function Description INPUT_AC_A Input Phase A voltage sensing forPLL INPUT_AC_B Input Phase B voltage sensing for PLL INPUT_AC_C InputPhase C voltage sensing for PLL IN_DC Input Bus voltage for thecapacitor OUT_RTO_PLUS_A Output Phase A positive RTO OUT_RTO_MINUS_AOutput Phase A negative RTO OUT_RTO_PLUS_B Output Phase B positive RTOOUT_RTO_MINUS_B Output Phase B negative RTO OUT_RTO_PLUS_C Output PhaseC positive RTO OUT_RTO_MINUS_C Output Phase C negative RTO OUT_PRECHARGEOutput Pre-charge relay control signal OUT_BLEEDER Output Bleeder relaycontrol signal OUT_SCR_PREFERRED Output Preferred side SCR gating signalOUT_SCR_ALTERNATE Output Alternate side SCR gating signal

FIG. 7 shows a flowchart of the method of transferring the power supplyfor the load 14 from one power source 12A to another power source 12B ina static transfer switch 10. As understood, the method may beimplemented by a controller 64, which may be in the form of a DSP 64. Insteps 66-68, the energy storage (capacitors 36) are precharged by thecharging circuit 46. The voltage of the DC bus 48 between the chargingcircuit 46 and the resonant circuits 44 is monitored and controlled tomaintain a desired charge on the capacitors 36. That is, the relays 56,58 are opened and closed as needed to supply voltage and bleed voltageto charge the capacitors 36. In step 70, the quality of the power beingsupplied by the power source 12A connected to the load 14 is monitoredfor degraded performance events. When such a situation is identified,the main switches 26 for the first power source 12A are opened todisconnect the first power source 12A from the load 14 in steps 72-78.That is, in step 72 turn off gate signals are sent to the main switches26. The auxiliary switches 34 that must be turned on in each resonantcircuit 44 are then determined in step 74 by phase locked loops 62 (seeOUT_RTO_MINUS, OUT_RTO_PLUS algorithm above). The auxiliary switches 34that have been determined in step 74 are then turned on with gatesignals in step 76.

In step 78, the DSP 64 verifies that the preferred power source 12A iscompletely disconnected by confirming that the net value of the currentand voltage passing through the main switches 22A, 26 for each phase ofthe preferred power source 12A is zero or negligible enough to confirmthat the resonant circuits 44 did in fact reverse the bias for each ofthe main circuits 42. Finally, the DSP 64 can initiate a turn on commandto the alternate power source 12B in step 80. Due to power qualityconsiderations like inrush and soft start and preferred turn onconditions, it is possible to vary the method used to turn on the secondpower source 12B while still making use of the improved method ofturning off the main switches 26 of the first power source 12A. Afterthe main switches 22B, 26 for the alternate power source 12 have beenturned on, the power transfer has been completed in step 82.

FIGS. 8-9 show a control system that may be used to control the resonantcircuits 44, and associated charging circuit 46. It is understood thatthe control system may also be used to control the main circuits 42 aswell. The Digital Signal Processor (DSP) 64 may be used to control allthe relays, sense the voltages for the PLL synchronization and controlthe main switches 26, 42 and RTO thyristors 34, 44. As shown, a scalingprocessor 84 may be provided to adjust the inputs for use by the DSP 64.As also shown in FIG. 9, the DSP 64 may include a PLL element 62 and aphase decoder element 86 to evaluate the current direction of each phaseand generate the gate signals for the auxiliary switches 34.

While preferred embodiments of the inventions have been described, itshould be understood that the inventions are not so limited, andmodifications may be made without departing from the inventions herein.While each embodiment described herein may refer only to certainfeatures and may not specifically refer to every feature described withrespect to other embodiments, it should be recognized that the featuresdescribed herein are interchangeable unless described otherwise, evenwhere no reference is made to a specific feature. It should also beunderstood that the advantages described above are not necessarily theonly advantages of the inventions, and it is not necessarily expectedthat all of the described advantages will be achieved with everyembodiment of the inventions. The scope of the inventions is defined bythe appended claims, and all devices and methods that come within themeaning of the claims, either literally or by equivalence, are intendedto be embraced therein.

The invention claimed is:
 1. A static transfer switch, comprising: a setof first power inputs coupled to a first three-phase electrical powersource, each of the first power inputs being coupled to one phase of thefirst three-phase electrical power source; a set of second power inputscoupled to a second three-phase electrical power source, each of thesecond power inputs being coupled to one phase of the second three-phaseelectrical power source; a set of power outputs coupled to a three-phaseelectrical load, each of the power outputs being coupled to one phase ofthe three-phase electrical load; a set of first switches coupled betweenthe set of first power inputs and the set of power outputs; a set ofsecond switches coupled between the set of second power inputs and theset of power outputs; a set of third switches coupled between a set ofenergy storages and the set of first switches, each third switch beingcoupled between one of the energy storages and a respective firstswitch; a sensor to determine an electrical property of the firstthree-phase electrical power source; a power transfer controller, thesensor being an input to the power transfer controller and the sets offirst and second switches being outputs of the power transfercontroller; a charge circuit to maintain a predetermined charge of theset of energy storages; and a fourth switch coupled between the set ofenergy storages and the charge circuit; wherein during normal operationthe power transfer controller closes the set of first switches toelectrically connect the set of first power inputs and the set of poweroutputs together and opens the set of second switches to electricallydisconnect the set of second power inputs and the set of power outputs,the first three-phase electrical power source thereby supplying power tothe three-phase electrical load; wherein when the sensor identifiesdegraded performance of the first three-phase electrical power source,the power transfer controller initiates a switching event, comprising:opening the fourth switch to stop charging of the set of energystorages; turning off a gate signal to each of the first switches;closing at least one third switch between each first switch and eachrespective energy storage after the gate signal of the respective firstswitch has been turned off, the set of energy storages thereby releasinga current to the input or output of the respective first switch to forcea drop in current conducted through the respective first switch, thedrop in current causing the respective first switch to open and stopconducting current therethrough between the first three-phase electricalpower source and the three-phase electrical load, the fourth switchbeing opened before the at least one third switches are closed; andclosing the set of second switches after respective first switches havebeen opened, the second three-phase electrical power source therebysupplying power to the three-phase electrical load.
 2. The statictransfer switch according to claim 1, wherein the switching event occurswithin one electrical cycle of the second three-phase electrical powersource.
 3. The static transfer switch according to claim 1, wherein theset of first switches comprise silicon controlled rectifiers.
 4. Thestatic transfer switch according to claim 1, wherein the set of firstswitches comprise integrated gate-commutated thyristors (IGCT), reverseblocking integrated gate-commutated thyristors (IGCT), or gate turn-offthyristors (GTO).
 5. The static transfer switch according to claim 1,wherein the set of third switches comprise thyristors.
 6. The statictransfer switch according to claim 1, wherein the set of power outputsare coupled to a transformer.
 7. The static transfer switch according toclaim 1, wherein the three-phase electrical load comprises a datacenter.
 8. The static transfer switch according to claim 1, wherein oneof the first and second three-phase electrical power sources comprisesan uninterruptible power supply (UPS).
 9. The static transfer switchaccording to claim 1, wherein each of the third switches is closed atthe same time.
 10. The static transfer switch according to claim 1,further comprising two of the third switches coupled to each of thefirst switches, one of the third switches being coupled to the input ofthe respective first switch and another of the third switches beingcoupled to the output of the respective first switch, the respectiveenergy storage being disposed between the two third switches.
 11. Thestatic transfer switch according to claim 1, further comprising two ofthe third switches coupled to each of the first switches, the two thirdswitches both being disposed between the respective energy storage andthe input or the output of the respective first switch, wherein one ofthe two third switches is closed if a positive current is beingconducted through the respective first switch and another of the twothird switches is closed if a negative current is being conductedthrough the respective first switch.
 12. The static transfer switchaccording to claim 1, further comprising four of the third switchescoupled to each of the first switches, two of the third switches beingcoupled to the input of the respective first switch and two of the thirdswitches being coupled to the output of the respective first switch, therespective energy storage being disposed between two of the thirdswitches on one side and two of the third switches on another side,wherein two of the third switches on opposite sides of the respectiveenergy storage are closed if a positive current is being conductedthrough the respective first switch, and a different two of the thirdswitches on opposite sides of the respective energy storage are closedif a negative current is being conducted through the respective firstswitch.
 13. The static transfer switch according to claim 1, furthercomprising an inductor coupled between the input or output of each firstswitch and the respective third switch.
 14. The static transfer switchaccording to claim 1, wherein the set of energy storages are capacitors.15. The static transfer switch according to claim 1, wherein the currentreleased by each of the energy storages and the current conductedthrough the respective first switch ceases to flow to the three-phaseelectrical load within 0.5 ms of turning off the gate signal to therespective first switch and closing the respective third switch.
 16. Thestatic transfer switch according to claim 1, wherein the set of firstswitches comprise silicon controlled rectifiers, further comprising fourof the third switches coupled to each of the first switches, two of thethird switches being coupled to the input of the respective first switchand two of the third switches being coupled to the output of therespective first switch, the respective energy storage being disposedbetween the two of the third switches on one side and two of the thirdswitches on another side, wherein two of the third switches on oppositesides of the respective energy storage are closed if a positive currentis being conducted through the respective third switch, and a differenttwo of the third switches on opposite sides of the respective energystorage are closed if a negative current is being conducted through therespective third switch, wherein the energy storages comprise a separatecapacitor coupled to each of the first switches.
 17. The static transferswitch according to claim 16, further comprising an inductor coupledbetween the input or output of each first switch and the respectivethird switch, and a set of the fourth switch with each fourth switchbeing coupled between one of the separate capacitors and the chargecircuit.
 18. The static transfer switch according to claim 1, furthercomprising a set of the fourth switch with each fourth switch beingcoupled between one of the energy storages and the charge circuit. 19.The static transfer switch according to claim 18, wherein the switchingevent occurs within one electrical cycle of the second three-phaseelectrical power source.
 20. The static transfer switch according toclaim 19, wherein the set of energy storages are capacitors.